The Marine Biogeochemistry of Dissolved and Colloidal Iron

Jessica Fitzsimmons, Ph.D., 2013Ed Boyle, Advisor

Iron is an essential micronutrient, which at the low concentrations found in marine waters can drive phytoplankton to iron limitation. Thus, understanding the sources, sinks, and cycling of dissolved Fe (dFe) was the focus of this thesis, with an emphasis on the role of the size partitioning of dFe (<0.2µm) into soluble (sFe<0.02µm) and colloidal (0.02µm<cFe<0.2µm) fractions.

In the North Atlantic, elevated dFe coincident with the oxygen minimum zone was determined to be caused by remineralization of high Fe:C organic material (vertical flux) instead of a lateral margin flux. In the South Pacific, correlated dFe, manganese, and He-3 maxima near 2000m suggested a hydrothermal source, proving that hydrothermal Fe can be transported 100s-1000s of kilometers away from its vent source in the dissolved phase.

In the North Atlantic, colloidal-dominated partitioning was observed in the dust-laden surface ocean, near the TAG hydrothermal system, and along the western margin. However, cFe was depleted or absent at the deep chlorophyll maximum. This data supports a model where "steady-state" dFe exchange between soluble and colloidal pools following remineralization modulates a constant partitioning of ~50% dFe into the colloidal phase throughout the subsurface North Atlantic, while in the upper ocean sFe and cFe cycle independently.